Yes, Pix.
I agree with you that T200 timer should be realized on A-bis interface between BSC and BTS. As well as on each separate Layer2 link (i.e. MS-BTS and BTS-BSC).
When I said about T200 I implied only MS-Network interface as in 04.06.
In general T200 had come to mobile network from ISDN Layer2 specification ETS 300 125.